The invention relates to a current-limiting circuit, which limits an electrical current from a voltage source to a consumer to a predetermined maximum current. Such a circuit includes a measuring resistor coupled into a current lead between a circuit input and a circuit output, a transistor, which is coupled into the current lead with its collector-emitter path in series with the measuring resistor and whose base is connected to the current lead through a series resistor, and a shunt regulator with an anode, a reference input and a cathode, wherein the cathode is connected to the base of the transistor, and the anode and the reference input form a voltage tap across the measuring resistor.
These types of circuits are frequently pointed out as typical examples of the application of shunt regulators, which are also called controllable diodes. Such circuits are also frequently called constant current sources although this designation is appropriate only when the power requirement of the consumer corresponds to or exceeds the predetermined maximum current. When the consumer has a lower power requirement, the circuit delivers only the lower, required current.
FIG. 2 shows a schematic representation of a generic circuit. The circuit 200 is suitable for limiting the current flow between a voltage source, which can be connected to circuit input 202, and a consumer, which can be connected to the circuit output 204, to a predetermined maximum current. This means that the circuit 200 lets any current required that is below the maximum current pass through. However, the circuit 200 limits the actually delivered current to the maximum current when the current requirements are higher, as could occur when there is a short circuit in the consumer, for example. This satisfies an important requirement for the power supply of consumers in an explosion-prone environment, because sparking or overheating of components due to excessive current flow can be reliably prevented in this manner.
The collector-emitter path of a transistor 206 is connected between the circuit input 202 and the circuit output 204. A series resistor 208, which is connected between the circuit input 202 and the base of the transistor 206, generates the base current on the collector-base path needed to control the transistor 206.
A measuring resistor 210 is connected downstream to the emitter of the transistor 206. Two branches 212, 214 of a voltage tap detect the voltage drop across the measuring resistor 210, wherein the branch 214, which is mounted downstream of the measuring resistor 210, is connected to the anode and the branch 212, which is mounted upstream of the measuring resistor 210, is connected to the reference input of a shunt regulator 216. The cathode of the shunt regulator 216 is connected to the base of the transistor 206. In the context of this application, a shunt regulator is generally understood to mean an electronic component that detects a voltage between its anode and its reference input and delivers a current correlated with the applied voltage to its cathode when a predefined reference voltage is exceeded. The cathode does not deliver any current below the predefined reference voltage. Such components, which are also known as controllable diodes, can be obtained, for example, from Semiconductor Components Industries, LLC under the designation TLV431A. Components with essentially the same effect can likewise be obtained from other manufacturers under other designations.
The voltage source delivers to the consumer the current required by the consumer below the allowable maximum current across the collector-emitter path of the transistor 206 and the measuring resistor 210. The shunt regulator 216 is not active in this type of normal operation, i.e., the base current of the transistor 206 is determined solely by the voltage at the circuit input 202 and by the series resistor 208. If a short circuit occurs in the consumer, the current through the transistor 206 and the measuring resistor 210 rises. The voltage drop across the measuring resistor 210 also grows. As soon as the voltage drop across the measuring resistor 210 exceeds the reference voltage of the shunt regulator 216, the shunt regulator connects through and its cathode extracts current from the base of the transistor 206. The transistor 206 therefore reduces the current flow on the collector-emitter path so far until the voltage drop across the measuring resistor 210 corresponds to the reference voltage of the shunt regulator 216. The current to the consumer is thereby limited to the allowable maximum current.
One disadvantage in this circuit is the high power loss at the transistor 206 in the event of a short circuit when the transistor must conduct maximum current. When supplying consumers in an explosion-prone environment, the power losses that occur in normal operation are in the range of at most ⅔ of the allowable maximum power. This means that a transistor optimally dimensioned for normal operation will be overloaded when conducting the maximum current in the event of a short circuit. Permanent overloading can result in destruction of the transistor, causing it to fully connect through and resulting in an impermissibly high current in the consumer. This can in turn lead to destruction of the consumer or to undesirable sparking or heating in the explosion-prone environment.
A remedy can be created by dimensioning the transistor large, which is not optimized for the power losses occurring in normal operation but for the maximum power. But this leads to disadvantages in the normal operation of the consumer, higher costs, and technical problems in mounting printed circuit boards, for example when the dimensions achievable in SMD construction are exceeded. Alternatively, the transistor can be protected with a thermocontroller that opens, e.g., a mechanical switch, when the transistor overheats. But this solution is expensive and exhibits an undesired time delay. In addition, such a circuit can as a rule be restored to its original state again only by specially trained specialists after the short circuit is resolved and therefore represents an annoyance for the user, especially for short-term short circuits such as occur when connecting a consumer.